1. Field of the Invention
The present invention relates to a broadband wireless access communication system, and more particularly to an apparatus and a method for selecting a serving base station according to a drop of a mobile subscriber station during a communication.
2. Description of the Related Art
Recently, extensive studies and research have been being carried out for the 4th generation (“4G”) communication systems in order to provide subscribers with services having a superior quality of service (“QoS”) at higher transmission speeds. In particular, studies are being actively carried out in relation to the 4G communication systems in order to provide high speed services having a superior QoS through broadband wireless access communication systems, such as wireless local area network (“LAN”) communication systems and wireless metropolitan area network (“MAN”) communication systems, while ensuring the mobility of the broadband wireless access communication systems.
The wireless MAN communication system has a wide service coverage area and provides data at a higher transmission speed than a LAN system, and as such the wireless MAN communication system is adaptable for a high-speed communication service. However, the wireless MAN communication system does not take into consideration the mobility of a user, that is, subscriber station (“SS”), so a handover, which is required when the SS moves at a high speed, is not taken into consideration in the wireless MAN communication system. The wireless MAN communication system is one type of broadband wireless access communication system and has a wider service coverage area and higher transmission speed as compared with those of a wireless LAN communication system.
In order to provide a broadband transport network for a physical channel of the wireless MAN communication system, an IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system utilizing an orthogonal frequency division multiplexing (“OFDM”) scheme and an orthogonal frequency division multiple access (“OFDMA”) scheme has been suggested.
As the IEEE 802.16a communication system applies the OFDM/OFDMA schemes to the wireless MAN system, the physical channel signals can be transmitted through a plurality of sub-carriers so that a high-speed data transmission is possible. In short, the IEEE 802.16a communication system is a broadband wireless access communication system using the OFDM/OFDMA schemes.
Hereinafter, a structure of a conventional IEEE 802.16a communication system will be described with reference to FIG. 1.
FIG. 1 is a structure diagram schematically illustrating the conventional IEEE 802.16a communication system.
Referring to FIG. 1, the IEEE 802.16a communication system has a single cell structure and includes a base station (BS) 100 and a plurality of SSs 110, 120 and 130 managed by the base station 100. The base station 100 conducts communications with the SSs 110, 120 and 130 using the OFDM/OFDMA schemes.
Hereinafter, a structure of a downlink frame of the IEEE 802.16a communication system will be described with reference to FIG. 2.
FIG. 2 is a structure diagram schematically illustrating the structure of the downlink frame of the IEEE 802.16a communication system.
Referring to FIG. 2, the downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of time division multiplex (“TDM”) fields 220 and 230. A synchronous signal, that is, a preamble sequence for synchronizing the SSs with the base station, is transmitted through the preamble field 200. The broadcast control field 210 includes a DL (downlink)_MAP field 211 and a UL (uplink)_MAP field 213. The DL_MAP field 211 is a field for transmitting a DL_MAP message. Information elements (“IEs”) included in the DL_MAP message are represented in Table 1.
TABLE 1SyntaxSizeNotes DL_MAP_Message_Format( ) { Management Message Type=2 8 bits PHY Synchronization FieldVariableSee Appropriate PHY specification     DCD Count 8 bits   Base Station ID48 bits Number of DL_MAP Element n16 bitsBegin PHY Specific section {See Applicable PHY section  for (i=1; i<=n; i++)For each DL_MAP element 1 to nDL_MAP Information Element( )VariableSee corresponding PHY specification  if!(byte boundary) { 4 bitsPadding to reach byte boundary   Padding Nibble       }      }    }  }
As shown in Table 1, the DL_MAP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, PHY (physical) Synchronization Field corresponding to modulation/demodulation schemes applied to a physical channel for achieving synchronization, DCD Count representing a count according to the variation of a configuration of a downlink channel descript (“DCD”) message including a downlink burst profile, Base Station ID, and Number of DL_MAP Elements n representing the number of elements remaining after the Base Station ID. Although it is not shown in Table 1, the DL_MAP message also includes information related to the ranging codes assigned to each ranging, which will be described later.
In addition, the UL_MAP field 213 is a field for transmitting a UL_MAP message. IEs included in the UL_MAP message are represented in Table 2.
TABLE 2SyntaxSizeNotes UL_MAP_Message_Format( ) { Management Message Type=3 8 bits   Uplink Channel ID 8 bits     UCD Count 8 bits Number of UL_MAP Element n16 bits  Allocation Start Time32 bitsBegin PHY Specific section {See Applicable PHY section   for (i=1; i<=n; i++)For each UL_MAP element 1 to nUL_MAP_Information_Element( )VariableSee corresponding PHY specification      }    }   }
As shown in Table 2, the UL_MAP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Uplink Channel ID representing an available uplink channel ID, UCD Count representing a count according to the variation of a configuration of an uplink channel descript (“UCD”) message including an uplink burst profile, and Number of UL_MAP Elements n representing the number of elements remaining after the UCD count. The Uplink Channel ID is allocated only to a medium access control (“MAC”) sub-layer.
The TDM field s 220 and 230 are field s corresponding to time slots which are allocated according to the TDM/TDMA (time division multiple access) schemes corresponding to the SSs. The base station transmits broadcast information to the SSs, which are managed by the base station, through the DL_MAP field 211 of the downlink frame by using a predetermined center carrier. As the SSs are powered on, the SSs monitor all frequency bands, which are preset in the SSs, in order to detect a reference channel signal, such as a pilot channel signal having the highest carrier to interference and noise ratio (“CINR”).
An SS selects a base station, which has transmitted to the SS the pilot signal having the highest CINR, as a base station for the SS. The SS can then recognize information controlling the uplink and the downlink of the SS and information representing a real data transmission/reception position by checking the DL_MAP field 211 and the UL_MAP field 213 of the downlink frame transmitted from the base station.
A configuration of the UCD message is represented in Table 3.
TABLE 3SyntaxSizeNotesUCD-Message_Format( ) { Management Message Type=08 bits Uplink channel ID8 bits Configuration Change Count8 bits Mini-slot size8 bits Ranging Backoff Start8 bits Ranging Backoff End8 bits Request Backoff Start8 bits Request Backoff End8 bits TLV Encoded Information for the overall channelVariable Begin PHY Specific Section {  for(i=1; i<n; i+n)   Uplink_Burst_DescriptorVariable  } }}
As shown in Table 3, the UCD message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Uplink channel ID representing an available uplink channel ID, Configuration Change Count counted in the base station, mini-time slot size representing a size of a mini-time slot of an uplink physical channel, Ranging Backoff start representing a start point of backoff using an initial ranging, that is, representing a size of an initial backoff window using the initial ranging, Ranging Backoff End representing an end point of backoff using an initial ranging, that is, representing a size of a final backoff window, Request Backoff start representing a start point of backoff for contention data and requests, that is, representing a size of an initial backoff window, and Request Backoff End representing an end point of backoff for contention data and requests, that is, representing a size of a final backoff window. A backoff value is a waiting time required for the next ranging if the present ranging fails. If the SS fails to perform the ranging, the base station must transmit the backoff value, i.e. the waiting time for the next ranging, to the SS. For instance, if the backoff value is determined as “10” based on the ranging backoff start and the ranging backoff end, the SS must perform the next ranging after by passing 210 ranging chances (1024 ranging chances) according to a truncated binary exponential backoff algorithm.
A structure of an uplink frame of the IEEE 802.16a communication system will be described with reference to FIG. 3.
FIG. 3 is a structure diagram schematically illustrating the structure of the uplink frame of the IEEE 802.16a communication system.
Prior to explaining FIG. 3, a description will be made in relation to rangings, such as an initial ranging, a maintenance ranging, that is, a periodic ranging, and a bandwidth request ranging, used for the IEEE 802.16a communication system.
First, the initial ranging will be described. The initial ranging is carried out in order to synchronize the base station with the SS, in which a time offset and a transmit power between the SS and the base station are precisely adjusted. That is, after the SS has been powered on, the SS receives the DL_MAP message and the UL_MAP/UCD message in order to synchronize with the base station. Then, the initial ranging is carried out with respect to the SS in order to adjust the time offset and the transmit power of the SS in relation to the base station. Herein, since the IEEE 802.16a communication system uses the OFDM/OFDMA schemes, ranging sub-channels and ranging codes are required for the initial ranging. Thus, the base station assigns available ranging codes to the SS according to the object or the type of rangings.
In detail, the ranging codes are created by segmenting a pseudo-random noise (“PN”) sequence having a predetermined bit length into predetermined ranging code units. In general, two ranging sub-channels, having a 53-bit length, forms one ranging channel and a PN code is segmented through a ranging channel having a 106-bit length, thereby forming the ranging codes. Such ranging codes are assigned to the SS, for instance, a maximum of 48 ranging codes (RC #1 to RC #48) can be assigned to the SS. At least two ranging codes are used for the initial ranging, the periodic ranging and the bandwidth request ranging as default values with respect to each SS. That is, the ranging codes are differently assigned according to the initial ranging, the periodic ranging and the bandwidth request ranging. For instance, N ranging codes are assigned for the initial ranging, M ranging codes are assigned for the periodic ranging, and L ranging codes are assigned for the bandwidth request ranging. As mentioned above, the assigned ranging codes are transmitted to the SS through the UCD message and the SS performs the initial ranging by using the ranging codes included in the UCD message in match with objects of the ranging codes.
Second, the periodic ranging will be described. The periodic ranging is periodically carried out by means of the SS having the time offset and the transmit power adjusted through the initial ranging, in such a manner that the SS can adjust the channel status with respect to the base station. The SS performs the periodic ranging by using ranging codes assigned thereto for the periodic ranging.
Third, the bandwidth request ranging will be described. The bandwidth request ranging is carried out by means of the SS having the time offset and the transmit power adjusted through the initial ranging, wherein the SS requests a bandwidth assignment in order to communicate with the base station.
Referring back to FIG. 3, the uplink frame consists of an initial maintenance opportunities field 300 using the initial ranging and the maintenance ranging, that is, the periodic ranging, a request contention opportunities field 310 using the bandwidth request ranging, and SS scheduled data fields 320 including uplink data of the SSs. The initial maintenance opportunities field 300 includes a plurality of access burst intervals including the real initial ranging and the periodic ranging and a collision interval created because of the collision between the access burst intervals. The request contention opportunities field 310 includes a plurality of bandwidth request intervals including the real bandwidth request ranging and a collision interval created because of the collision between the bandwidth request intervals. In addition, the SS scheduled data fields 320 consist of a plurality of SS scheduled data fields (first SS scheduled data field to SS Nth scheduled data field) and SS transition gaps formed between the SS scheduled data fields (first SS scheduled data field to SS Nth scheduled data field).
An uplink interval usage code (“UIUC”) field is provided for recoding information representing the usage of the offset recorded in the offset field. The UIUC field is shown in Table 4.
TABLE 4ConnectionIE nameUIUCIDDescriptionreserved0NAReserved for future use.Request1anyStarting offset of request region.Initial2broadcastStarting offset of maintenance region (used in Initial Ranging).MaintenanceStation3unicastStarting offset of maintenance region (used in periodic Ranging).MaintenanceData Grant4unicastStarting offset of Data Grant Burst Type 1 assignment.Burst Type 1Data Grant5unicastStarting offset of Data Grant Burst Type 2 assignment.Burst Type 2Data Grant6unicastStarting offset of Data Grant Burst Type 3 assignment.Burst Type 3Data Grant7unicastStarting offset of Data Grant Burst Type 4 assignment.Burst Type 4Data Grant8unicastStarting offset of Data Grant Burst Type 5 assignment.Burst Type 5Data Grant9unicastStarting offset of Data Grant Burst Type 6 assignment.Burst Type 6Null IE10zeroEnding offset of the previous grant.Used to bound the length of the last actual interval allocation.Empty11zeroUsed to schedule gaps in transmission.reserved12-15N/AReserved.
As shown in Table 4, if “2” is recorded in the UIUC field, the starting offset used for the initial ranging is recorded in the offset field. If “3” is recorded in the UIUC field, the starting offset used for the bandwidth request ranging or the maintenance ranging is recorded in the offset field. As mentioned above, the offset field is provided to record starting offset values used for the initial ranging, the bandwidth request ranging or the maintenance ranging corresponding to information recorded in the UIUC field. Information related to the characteristics of a physical channel transmitted from the UIUC field is recorded in the UCD.
A ranging process between the base station and the SS in the IEEE 802.16a communication system will be described with reference to FIG. 4.
FIG. 4 is a signal flow diagram illustrating the ranging process between the base station and the SS in the IEEE 802.16a communication system.
Referring to FIG. 4, as an SS 400 is powered on, the SS 400 monitors all of the frequency bands, which are preset in the SS 400, in order to detect a pilot channel signal having the highest CINR. In addition, the SS 400 selects a base station 420 which has transmitted the pilot signal having the highest CINR to the SS 400 as a base station for the SS 400, so the SS 400 receives the preamble of the downlink frame transmitted from the base station 420, thereby obtaining system synchronization with respect to the base station 420.
As described above, when the system synchronization is attained between the SS 400 and the base station 420, the base station 420 transmits the DL_MAP message and the UL_MAP message to the SS 400 (steps 411 and 413). Herein, as described above with reference to Table 1, the DL_MAP message notifies the SS 400 of the information required for the SS 400 to obtain the system synchronization with respect to the base station 420 in the downlink and information about a structure of the physical channel capable of receiving messages transmitted to the SS 400 from the downlink. In addition, as describe above with reference to Table 2, the UL_MAP message notifies the SS 400 of the information about a scheduling period of the SS 400 in the uplink and the structure of the physical channel. In addition, the DL_MAP message is periodically broadcast to all of the SSs from the base station 420. If a predetermined SS, that is, if the SS 400 can continuously receive the DL_MAP message, it will be represented that the SS 400 is synchronized with the base station 420. That is, the SS 400 receiving the DL_MAP message can receive all of the messages transmitted to the downlink. In addition, as described above with reference to Table 3, if the SS 400 fails to access to the base station 420, the base station 420 transmits the UCD message including the information representing the available backoff value to the SS 400.
The SS 400, which has been synchronized with the base station 420, transmits a ranging request (“RNG_REQ”) message to the base station 420 (step 415). Upon receiving the RNG_REQ message from the SS 400, the base station 420 transmits a ranging response (“RNG_RSP”) message including information required for correcting frequency for the ranging, time and transmit power to the SS 400 (step 417).
A configuration of the RNG_REQ message is represented in Table 5.
TABLE 5SyntaxSizeNotesRNG-REQ_Message_Format( ) { Management Message Type = 48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
In Table 5, the “Downlink Channel ID” is a downlink channel identifier included in the RNG_REQ message received in the SS through the UCD and the “Pending Until Complete” is a priority of transmitted ranging responses. If the “Pending Until Complete” is “0”, a previously transmitted ranging response has a priority, and if the “Pending Until Complete” is not “0”, a presently transmitted ranging response has a priority.
A configuration of the RNG_RSP message is represented in Table 6.
TABLE 6SyntaxSizeNotesRNG-RSP_Message_Format( ) { Management Message Type = 58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
In Table 6, the “Uplink Channel ID” is an ID of an uplink channel included in the RNG_REQ message. Since the IEEE 802.16a communication system shown in FIG. 4 relates to a fixed SS, that is, since the IEEE 802.16a communication system shown in FIG. 4 does not take into consideration the mobility of the SS, the base station 420 communicating with the SS 400 becomes a serving base station.
The IEEE 802.16a communication system has the signal cell structure in which the mobility of the SS is not considered. Meanwhile, an IEEE 802.16e communication system is defined as a communication system in which the mobility of the SS is added to the IEEE 802.16a communication system. Thus, the IEEE 802.16e communication system must consider the mobility of the SS under a multi-cell environment. In order to ensure the mobility of the SS under the multi-cell environment, the operations of the SS and the base station must be changed. To this end, various studies have been carried out relating to a handover of the SS in order to provide for the mobility to the SS under the multi-cell environment.
A structure of a conventional IEEE 802.16e communication system will be described with reference to FIG. 5.
FIG. 5 is a structure diagram schematically illustrating the structure of the conventional IEEE 802.16e communication system.
Referring to FIG. 5, the IEEE 802.16e communication system has a multi-cell structure consisting of cells 500 and 550 and includes a first base station 510 for managing the cell 500, a second base station 540 for managing the cell 550, and a plurality of mobile subscriber stations (“MSSs”) 511, 513, 530, 551, and 553. The MSS signify an SS having mobility. The base stations 510 and 540 communicate with the MSSs 511, 513, 530, 551, and 553 using the OFDM/OFDMA schemes. From among the MSSs 511, 513, 530, 551, and 553, the MSS 530 is positioned in a boundary cell formed between the cell 500 and the cell 550, that is, the MSS 530 is positioned in a handover region. Thus, the MSS 530 must be provided with a handover function in order to realize the mobility of the MSS 530.
In the IEEE 802.16e communication system, a MSS receives pilot channel signals transmitted from a plurality of base stations and measures the CINR of the pilot channel signals. In addition, the MSS selects a base station, which has transmitted a pilot signal having a highest CINR, as a base station of the MSS. That is, the MSS regards the base station transmitting the pilot signal having the highest CINR as a serving base station of the MSS. After selecting the serving base station, the MSS receives the downlink frame and the uplink frame transmitted from the serving base station. Herein, the downlink frame and the uplink frame of the IEEE 802.16e communication system have structures identical to those of the downlink frame and the uplink frame of the IEEE 802.16a communication system described with reference to FIGS. 2 and 3.
The serving base station transmits a mobile neighbor advertisement (“MOB_NBR_ADV”) message to the MSS. A configuration of the MOB_NBR_ADV message is represented in Table 7.
TABLE 7SyntaxSizeNotesMOB_NBR-ADV_Message_Format( ) { Management Message Type = 48 8 bits Configuration Change Count 8 bits N_NEIGHBORS 8 bits For (j=0 : j<N_NEIGHBORS : j++) {  Neighbor BS-ID48 bits  Physical Frequency32 bits  TLV Encoded Neighbor informationVariableTLV specific }}
As shown in Table 7, the MOB_NBR_ADV message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Configuration Change Count representing the number of configurations to be changed, N_NEIGHBORS representing the number of neighbor base stations, Neighbor BS-ID representing identifiers of neighbor base stations, Physical Frequency representing a physical channel frequency of the neighbor base stations, and TLV (type length variable) Encoded Neighbor Information representing variable information about the neighbor base stations.
After receiving the MOB_NBR_ADV message, the MSS transmits a mobile scanning interval allocation request (“MOB_SCN_REQ”) message to the serving base station if it is necessary to scan the CINRs of the pilot channel signals transmitted from the neighbor base stations. A scan request time of the MSS for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. A configuration of the MOB_SCN_REQ message is represented in Table 8.
TABLE 8SyntaxSizeNotesMOB_SCN-REQ_Message_Format( ) { Management Message Type = ? 8 bits Scan Duration16 bitsUnits are frames.}
As shown in Table 8, the MOB_SCN_REQ message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, and Scan Duration representing a scan interval for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations. The scan duration is formed in a frame unit. In Table 8, the Management Message Type for the MOB_SCN_REQ message is not yet defined (Management Message Type=undefined).
After receiving the MOB_SCN_REQ message, the serving base station transmits a mobile scanning interval allocation response (“MOB_SCN_RSP”) message including scan information, which must be scanned by the MSS, to the MSS. A configuration of the MOB_SCN_RSP message is represented in Table 9.
TABLE 9SyntaxSizeNotesMOB_SCN-RSP_Message_Format( ) { Management Message Type = ? 8 bits Length 8 bitsin bytes For (i=0 : i<Length/3: i++) {  CID16 bitsbasic CID ofthe MSS  Duration 8 bitsin frames }}
As shown in Table 9, the MOB_SCN_RSP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, Connection ID (“CID”) of the MSS, which has transmitted the MOB_SCN_REQ message and Duration. In Table 9, the Management Message Type for the MOB_SCN_RSP message is not yet defined (Management Message Type=undefined). The Duration represents an area in which the MSS scans the CINR of the pilot channel signal. After receiving the MOB_SCN_RSP message including the scanning information, the MSS scans the CINRs of the pilot signals of the neighbor base stations included in the MOB_SCN_RSP message corresponding to scanning information parameters.
In order to provide the handover function in the IEEE 802.16e communication system, the MSS must measure the CINRs of the pilot channel signals transmitted from the neighbor base stations and the base station of the MSS, that is, the serving base station. If the CINR of the pilot channel signals transmitted from the serving base station is less than the CINRs of the pilot channel signals transmitted from the neighbor base stations, the MSS sends a signal requesting the handover to the serving base station.
A handover process according to the request of the MSS in the conventional IEEE 802.16e communication system will be described with reference to FIG. 6.
FIG. 6 is a signal flow diagram illustrating the handover process according to the request of the MSS in the conventional IEEE 802.16e communication system.
Referring to FIG. 6, a serving base station 610 transmits an MOB_NBR_ADV message to an MSS 600 (step 611). Upon receiving the MOB_NBR_ADV message from the serving base station 610, the MSS 600 obtains information related to the neighbor base stations and transmits an MOB_SCN_REQ message to the serving base station 610 if it is necessary to scan (“scan” and “measure” will be used synonymously with respect to determining CINRs) the CINRs of pilot channels signals transmitted from the neighbor base stations (step 613). A scan request time of the MSS 600 for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. The serving base station 610 receiving the MOB_SCN_REQ message transmits an MOB_SCN_RSP message including scanning information, which must be scanned by the MSS 600, to the MSS 600 (step 615). Upon receiving the MOB_SCN_RSP message including scanning information from the serving base station 610, the MSS 600 scans parameters included in the MOB_SCN_RSP message, that is, the MSS 600 scans the CINRs of the pilot channel signals of the neighbor base stations obtained through the MOB_NBR_ADV message (step 617). Although a process for measuring the CINR signal of the pilot channel signal transmitted from the serving base station 610 is not separately illustrated in FIG. 6, the MSS 600 may continuously measure the CINR of the pilot channel signal transmitted from the serving base station 610.
After scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations, if the MSS 600 decides to change the serving base station thereof (step 619), that is, if the MSS 600 decides to replace the serving base station 610 with a new base station having a structure different from the structure of the serving base station 610, the MSS 600 transmits a mobile MSS handover request (“MOB_MSSHO_REQ”) message to the serving base station 610. Herein, a base station, which can be selected as the new base station due to the handover of the MSS 600, is called a “target BS”. A configuration of the MOB_MSSHO_REQ message is represented in Table 10.
TABLE 10SyntaxSizeNotesMOB_MSSHO-REQ_Message_Format( ) { Management Message Type = 52 8 bits N_Recommended 8 bits For (j=0 : j<N_NEIGHBORS : j++) {  Neighbor BS-ID48 bits  BS S/(N+1) 8 bits  Service level prediction 8 bits }}
As shown in Table 10, the MOB_MSSHO_REQ message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, and a scanning result of the MSS 600. In Table 10, N_Recommended represents the number of neighbor base stations, which have transmitted pilot channel signals having CINRs greater than a predetermined CINR, detected through the scanning operation of the MSS 600 for the CINRs of the pilot channel signals transmitted from the neighbor base station. That is, the N_Recommended represents the number of neighbor base stations capable of performing the handover for the MSS 600. The MOB_MSSHO_REQ message also includes identifiers of the neighbor base stations represented by the N_Recommended, the CINRs of the pilot signals transmitted from the neighbor base stations, and a service level expected to be transmitted to the MSS 600.
The serving base station 610 receives the MOB_MSSHO_REQ message transmitted from the MSS 600 and detects a list of target base stations allowing the handover of the MSS 600 based on N_Recommended information of the MOB_MSSHO_REQ message (step 623). In the following description, the list of target base stations allowing the handover of the MSS will be referred to as a “handover-support target base station list” for the purpose of convenience. According to FIG. 6, a first target base station 620 and a second target base station 630 may exist in the handover-support target base station list. Of course, the handover-support target base station list may include a plurality of target base stations. The serving base station 610 transmits a handover notification (“HO_notification”) message to the target base stations included in the handover-support target base station list, such as the first target base station 620 and the second target base station 630 (steps 625 and 627). A configuration of the HO_notification message is represented in Table 11.
TABLE 11FieldSizeNotesGlobal Header152-bitFor (j=0): j<Num Records: j++) { MSS unique identifier 48-bit48-bit unique identifier used by MSS (as provided by theMSS or by the I-am-host-of message) Estimated Time to HO 16-bitIn milliseconds, relative to the time stamp, value 0 of thisparameter indicates that no actual HO is pending Required BW 8-bitBandwith which is required by MSS (to gurarantee minimumpacket data transmission) Required OoS 8-bitName of Service Class representing AuthorizedQoSParam-Set}Security fieldTBDA means to authenticate this messageCRC field 32-bitIEEE CRC-32
As shown in Table 11, the HO_notification message includes a plurality of IEs, such as an ID of the MSS 600 to be handed-over to the first target base station 620 or the second target base station 630, an expected handover start time of the MSS 600, a bandwidth provided from the target base station, that is, a bandwidth provided from a new serving base station according to a request of the MSS 600, and a service level provided to the MSS 600. The bandwidth and the service level requested by the MSS 600 are identical to the expected service level information recorded in the MOB_MSSHO_REQ message described with reference to FIG. 10.
The first and second target base stations 620 and 630 receive the HO_notification message from the serving base station 610 and transmit an HO_notification response message to the serving base station 610 (steps 629 and 631). A configuration of the HO_notification response message is represented in Table 12.
TABLE 12FieldSizeNotesGlobal Header152-bitFor (j=0: j<Num Records:j++) { MSS unique 48-bit48-bit unique identifier used by MSS (as identifierprovided by the MSS or by theI-am-host-of message) QoS 8-bitBandwidth which is provided by BS (to Estimatedguarantee minimum packet data transmission)TBD how to set this field BW Estimated 8-bitQuality of Service level Unsolicited Grant Service (UGS) Real-time Polling Service (rtPS) Non-real-time Polling Service (nrtPS) Best Effort ACK/NACK 1-bitAcknowledgement or Negativeacknowledgement 1 is Acknowledgement which means that the  neighbor BS accepts the HO-notification  message from the serving BS 0 is Negative acknowledgement which means  that the neighbor BS may not accept the   HO-notification message from the serving  BS}Security fieldTBDA means to a authenticate this messageCRC field 32-bitIEEE CRC-32
As shown in Table 12, the HO_notification response message includes a plurality of IEs, such as an ID of the MSS 600 to be handed-over to the target base stations, ACK/NACK representing a response of the target base stations with regard to a handover request of the MSS 600, and information related to the bandwidth and the service level which must be provided from each target base station when the MSS 600 is handed-over to the target base station.
The serving base station 610 receives the HO_notification response message from the first and second target base stations 620 and 630 and analyzes the HO_notification response message in order to select a final base station capable of providing an optimal bandwidth and an optimal service level to the MSS 600 when the MSS 600 is handed-over to the base station. For instance, if the service level provided from the first target base station 620 is less than the service level requested by the MSS 600 and the service level provided from the second target base station 630 is identical to the service level requested by the MSS 600, the serving base station 610 selects the second target base station 630 as the final target base station performing a handover operation in relation to the MSS 600. Thus, the serving base station 610 transmits an HO_notification conform message to the second target base station 630 in response to the HO_notification response message (step 633). A configuration of the HO_notification confirm message is represented in Table 13.
TABLE 13FieldSizeNotesGlobal Header152-bitFor (j=0: j<Num Records;j++) { MSS unique 48-bit48-bit universal MAC address of the MSS identifier(as provided to the BS on the RNG-REQmessage) QoS Estimated 8-bitBandwidth which is provided by BS(to guarantee minimum packet datatransmission) TBD how to set this field BW Estimated 8-bitQuality of Service level Unsolicited Grant Service (UGS) Real-time Polling Service (rtPS) Non-real-time Polling Service (nrtPS) Best Effort Service (BE)}Security fieldTBDA means to authenticate this messageCRC field 32-bitIEEE CRC-32
As shown in Table 13, the HO_notification confirm message includes a plurality of IEs, such as an ID of the MSS 600 to be handed-over to the selected target base station, and information about a bandwidth and a service level which must be provided from the selected target base station when the MSS 600 is handed-over to the selected target base station.
In addition, the serving base station 610 transmits a mobile handover response (“MOB_HO_RSP”) message to the MSS 600 in response to the MOB_MSSHO_REQ message (step 635). The MOB_HO_RSP message includes information about the target base station performing the handover operation in relation to the MSS 600. A configuration of the MOB_HO_RSP message is represented in Table 14.
TABLE 14SyntaxSizeNotesMOB_HO-RSP_Message_Format( ) { Management Message Type = 53 8 bits Estimated HO time 8 bits N_Recommended 8 bits For (j=0 : j<N_NEIGHBORS : j++) {  Neighbor BS-ID48 bits service level prediction 8 bitsThis parameter existsonly when the messageis sent by the BS }}
As shown in Table 14, the MOB_HO_RSP message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, an expected handover start time, and target serving stations selected from the serving base stations. In addition, N_Recommended of the MOB_HO_RSP message represents the number of target base stations capable of providing the bandwidth and service level requested by the MSS 600 among target base stations included in the handover-support target base station list. The MOB_HO_RSP message is marked with the identifiers of the target base stations represented by the N_Recommended and a service level expected to be provided to the MSS 600 from the target base station. Although FIG. 6 illustrates that the information of one target base station, that is, information about the second target base station 630, is only included in the MOB_HO_RSP message from among the target base stations included in the handover-support target base station list, if there are a plurality of target base stations capable of providing the bandwidth and service level requested by the MSS in the handover-support target base station list, the MOB_HO_RSP message may include information related to the plurality of target base stations.
Upon receiving the MOB_HO_RSP message, the MSS 600 analyzes information included in the MOB_HO_RSP message in order to select a target base station for performing the handover operation in relation to the MSS 600. After selecting the target base station, the MSS 600 transmits a mobile handover indication (“MOB_HO_IND”) message to the serving base station 610 in response to the MOB_HO_RSP message (step 637). A configuration of the MOB_HO_IND message is represented in Table 15.
TABLE 15SyntaxSizeNotesMOB_HO_IND_Message_Format( ) { Management Message Type = 54 8 bits TLV Encoded InformationVariableTLV specific Target_BS_ID48 bits}
As shown in Table 15, the MOB_HO_IND message includes a plurality of IEs, such as Management Message Type representing a message type to be transmitted, an identifier of a final target base station selected by the MSS 600, and TLV Encoded Information representing variable Encoded information.
The serving base station 610 receiving the MOB_HO_IND message recognizes that the MSS 600 will be handed-over to the target base station, that is, the second target base station 630 based on the MOB_HO_IND message so that the serving base station 610 releases a link connecting the serving base station 610 to the MSS 600 (step 639). If the link connecting the MSS 600 to the serving base station 610 has been released, the MSS 600 is handed-over to the second target base station 630.
The handover process according to the request of the base station in the conventional IEEE 802.16e communication system will be described with reference to FIG. 7.
FIG. 7 is a signal flow diagram showing the handover process according to the request of the base station in the conventional IEEE 802.16e communication system.
It is noted that the handover process according to the request of the base station may occur when an overload is applied to the base station so that it is necessary to distribute the load of the base station to neighbor base stations or when it is necessary to deal with the status variation of the uplink of the MSS.
Referring to FIG. 7, a serving base station 710 transmits an MOB_NBR_ADV message to an MSS 700 (step 711). Upon receiving the MOB_NBR_ADV message from the serving base station 710, the MSS 700 obtains information relating to the neighbor base stations and transmits an MOB_SCN_REQ message to the serving base station 710 if it is necessary to scan the CINRs of pilot channels signals transmitted from the neighbor base stations (step 713). A scan request time of the MSS 700 for scanning the CINRs of the pilot channel signals transmitted from the neighbor base stations does not directly relate to the CINR scanning operation, so it will not be further described below. The serving base station 710 receiving the MOB_SCN_REQ message transmits an MOB_SCN_RSP message including the scanning information, which must be scanned by the MSS 700, to the MSS 700 (step 715). Upon receiving the MOB_SCN_RSP message including the scanning information from the serving base station 710, the MSS 700 scans the parameters included in the MOB_SCN_RSP message, that is, the MSS 700 scans the CINRs of the pilot channel signals of the neighbor base stations obtained through the MOB_NBR_ADV message (step 717). Although a process for measuring the CINR signal of the pilot channel signal transmitted from the serving base station 710 is not separately illustrated in FIG. 7, the MSS 700 may continuously measure the CINR of the pilot channel signal transmitted from the serving base station 710.
When the serving base station 710 determines that it is necessary to perform the handover of the MSS 700 managed by the serving base station 710 (step 719), the serving base station 710 transmits an HO_notification message to the neighbor base stations (steps 721 and 723). Herein, the HO_notification message includes information relating to a bandwidth and a service level which must be provided from a target base station, that is, a new serving base station of the MSS 700. In FIG. 7, the neighbor base stations of the serving base station 710 are first and second target base stations 720 and 730.
Upon receiving the HO_notification message, the first and second target base stations 720 and 730 transmit the HO_notification response message to the serving base station 710 in response to the HO_notification message (step 725 and 727). As described with reference to Table 12, the HO_notification response message includes ACK/NACK representing a response of the target base stations, that is, a response of the neighbor base stations with regard to the handover requested by the serving base station 710, and information about a bandwidth and a service level of the target base stations, which must be provided to the MSS 700.
The serving base station 710 receives the HO_notification response message from the first and second target base stations 720 and 730 and selects the target base stations capable of providing an optimal bandwidth and an optimal service level to the MSS 700. For instance, if the service level provided from the first target base station 720 is less than the service level requested by the MSS 700 and the service level provided from the second target base station 730 is identical to the service level requested by the MSS 700, the serving base station 710 selects the second target base station 730 as a final target base station performing a handover operation in relation to the MSS 700. Thus, the serving base station 710 selecting the second target base station 730 as a final target base station transmits an HO_notification conform message to the second target base station 730 in response to the HO_notification response message (step 729).
The serving base station 710 transmits the MOB_HO_RSP message to the MSS 700 (step 731) after transmitting the HO_notification conform message to the second target base station 730. The MOB_HO_RSP message includes N_Recommended information selected by the serving base station 710, that is, information related to the bandwidth and the service level which must be provided to the MSS 700 from the selected target base stations (the second target base station 730 in FIG. 7) and target base stations. Upon receiving the MOB_HO_RSP message, the MSS 700 recognizes that the handover is requested by the serving base station 710 so that the MSS 700 selects the final target base station performing the handover operation in relation to the MSS 700 based on N_Recommended information included in the MOB_HO_RSP message. After that, the MSS 700 transmits the MOB_HO_IND message to the serving station 710 in response to the MOB_HO_RSP message (step 733). As the MOB_HO_IND message is received in the serving base station 710, the serving base station 710 recognizes that the MSS 700 will be handed-over to the target base station based on the MOB_HO_IND message so that the serving base station 710 releases a link connecting the serving base station to the MSS 700 (step 735). If the link connecting the MSS 700 to the serving base station 710 has been released, the MSS 700 is handed-over to the second target base station 730.
As described above, according to the conventional IEEE 802.16e communication system, the MSS is handed-over to the neighbor base station. The MSS is handed-over to the target base station, which is different from the serving base station, when the CINR of the pilot cannel signal of the serving base station becomes reduced so that it is impossible for the MSS to properly communicate with the serving base station, or when the handover is requested by the MSS or the serving base station. However, if an MSS drop occurs during the handover operation in the conventional IEEE 802.16e communication system, the MSS monitors all of the frequency bands in a similar way as to the operation of the MSS after the MSS is powered on in order to detect a pilot channel signal having the highest CINR and selects the base station, which has transmitted the pilot channel signal halving the highest CINR, as a base station for the MSS. In addition, if an MSS drop occurs while the MSS is communicating with the serving base station in the conventional IEEE 802.16e communication system, the MSS monitors all of the frequency bands in the same manner as the MSS drop so as to detect a pilot channel signal having the highest CINR and selects the base station, which has transmitted the pilot channel signal having the highest CINR, as a base station for the MSS.
According to the above two cases, the MSS monitors all of the frequency bands although the MSS is communicating with the serving base station, requiring a relatively long period of time for selecting the serving base station, thereby lowering service quality. Therefore, it is necessary to provide an improved procedure capable of allowing the MSS subject to the drop during a communication to resume communication with a minimum time delay.